Electromagnetic signals may carry information from one location to another. For example, signals may be sent and received between two electronic devices that are processing information on a circuit within a device. Such signals may be sent and received as voltage, current, light, magnetic fields, or as electric fields, for example. In some systems, signals may be sent over great distances, for example, by transmitting and receiving the signals in the form of propagating electromagnetic waves.
There are many applications in which signals may carry information from one location to another. An exemplary application is an electro-optic modulator that may be used to modulate an optical signal in, for example, an optical communication system. In an electro-optic modulator, an optical signal may be modulated in response to modulation in an electrical field passing through a medium in which the optical signal is propagating. In some electro-optic modulators, a microstrip waveguide may be used to provide electric fields oriented to pass through a medium. The input optical signal may be split into two paths through the medium using, for example, a Mach-Zehnder configuration. In certain media, electric fields may induce a relative phase shift between optical signals in the split paths. At the output of the electro-optic modulator, the split signals are re-combined. As such, the amplitude of the output optical signal is a function of the applied electric fields.
The electric fields may in turn be controlled by an electric signal that is transmitted to the electro-optic modulator through a transmission line. In some applications, the electric signal is transmitted through transmission lines that have electric field orientations that are not directly compatible with the electric field orientation used in a particular electro-optic modulator, which may be a microstrip waveguide. For example, an electric signal may be transmitted from a signal generator to a microstrip waveguide through a transmission line that includes coaxial and/or a coplanar waveguide sections.
Some applications may use one or more types of transmission lines to transport signals over a conductive signal path. Examples of transmission line types include coaxial cables, coplanar waveguides, microstrip waveguides and stripline waveguides. As a signal propagates through a transmission line, the signal has associated with it electric and magnetic fields. In each type of transmission line, the electric and/or magnetic fields may typically have a characteristic orientation. To transport a signal through more than one type of transmission line, some systems may provide transitions at the interfaces between different types of transmission lines. The interfaces may be designed to reduce or avoid abrupt changes in characteristic impedance that can cause signal loss.
In some applications, a forward and a return conductive path may provide a preferred low impedance current path between the source and the receiver. In some multilayer configurations, a coplanar return conductor on one layer may be electrically connected through vias to a microstrip return conductor on a different layer. In transmission lines and the interfaces between transmission lines, the geometries and properties of the forward and return signal paths, as well as the properties of the surrounding media, may determine the characteristic impedance. One technique for transitioning between coplanar and microstrip waveguides involves tapering geometries in the forward and return conductive paths.